1. Field of the Invention
The invention pertains generally to electronic circuits. In particular, it pertains to charge pump circuits.
2. Description of the Related Art
A charge pump provides an output voltage that is higher than its own supply voltage. Flash memories can use charge pumps to produce erase and program voltages.
FIG. 1A shows a schematic of a simple charge pump circuit 1, with two-stage charge pump 11 driving load 12. This figure shows only capacitors C1-C2 and diodes D1-D3 to produce voltages V1, V2, and V3, with resistor RL and capacitor CL providing the load, although other components could be added to the circuit for improved performance. FIG. 2B shows the various waveforms that are produced by this circuit. The operation of charge pumps is well known, and no further description of the circuit is provided herein.
The effectiveness of a charge pump is dependent on the frequency of the clock source, since the clock cycle affects the amount of charging and discharging that takes place in the capacitors.
FIG. 2 shows a charge pump control circuit 2. A voltage controlled oscillator (VCO) 22 provides the clock source, with its frequency being controlled by the output of differential amplifier 21. Clock drivers 23 convert the single VCO output to the multiple clocks required to drive multi-stage charge pump circuit 24. The voltage VOUT produced by charge pump circuit 24 can be sampled by voltage divider 25, which feeds back a pre-determined fraction of VOUT as voltage VFDBK. This is compared with a stable reference voltage VREF by differential amplifier 21, and the difference between VREF and VFDBK controls the output of differential amplifier 21, which in turn controls the frequency of the VCO clock. This closed loop circuit regulates the output of the charge pumps by controlling the frequency of the clocks that operate the charge pumps. Under a given set of conditions, every charge pump circuit has an optimum frequency that produces the maximum amount of current available from the circuit.
Unfortunately, the charge pumps and the regulation circuitry are subject to variations due to both temperature changes during operation and process variations during manufacture. Typically, for a given frequency from VCO 22, the maximum current from the charge pump circuit changes with changing temperature, so that the circuit must be overdesigned to handle the expected current demands at the worst case temperature. And process variations during manufacture can result in a circuit that is not optimized at any temperature.
FIG. 3 shows a graph of the operating characteristics of a typical charge pump circuit. The x-axis measures the VCO bias level (the voltage level at the input of the VCO), while the y-axis measures the corresponding output current, in micro-amps, that the charge pump circuit can produce. The dotted line shows the characteristics of the circuit at a temperature of 100 degrees C. For this example, the available output current is fairly constant with a bias level of up to 0.5 volts, but beyond 0.5 volts the output current drops off sharply, making the optimum bias voltage about 0.5 volts or slightly less.
The solid line shows the same curve for a temperature of xe2x88x9240 degrees C. The entire curve is shifted to the left by a significant amount, with the optimum bias level at about 0.3 volts. From this chart, it can be seen that a higher temperature requires a higher bias voltage, if the maximum current is to be available from the circuit at all operational temperatures.
Unfortunately, conventional circuits do not provide this adjustment, and the circuits must be designed for worst case conditions. This results in overdesign, which is more expensive and wasteful of circuit resources.